Field of the Invention
The present invention relates to a method for producing a Group III nitride semiconductor light-emitting device characterized by the structure of light-emitting layer. More specifically, the light-emitting layer has a MQW structure in which a plurality of layer units are repeatedly deposited, each layer unit comprising a well layer, a protective layer, and a barrier layer sequentially deposited, and is characterized by the method for forming a protective layer.
Background Art
A MQW structure comprising an InGaN well layer and an AlGaN barrier layer alternately and repeatedly deposited, is widely used as a light-emitting layer of Group III nitride semiconductor light-emitting device. Since the barrier layer is formed of AlGaN, the growth temperature of the barrier layer must be higher than that of the well layer to grow with good crystallinity. Therefore, after the formation of the well layer, it is necessary to raise the temperature, and then grow the barrier layer. However, In is evaporated from the well layer due to heating, thereby causing reduction in emission performance or variation in emission wavelength. Therefore, a protective layer is provided between the well layer and the barrier layer, which is grown at the same temperature as the growth temperature of the well layer, thereby preventing the evaporation of In.
Japanese Patent Application Laid-Open (kokai) No. 2010-80619 discloses that a protective layer formed of AlGaN having a thickness of 6 Å is used. Japanese Patent Application Laid-Open (kokai) No. 2012-216751 discloses that a protective layer formed of GaN having a thickness of 6 Å is used.
Moreover, Japanese Patent Application Laid-Open (kokai) No. 2001-332763 discloses that a protective layer is formed of InGaN, and the growth rate of the protective layer is 1.5 times to 5 times higher than that of the well layer. During heating for formation of the barrier layer after the formation of the protective layer, the protective layer is thermally decomposed so that the protective layer is almost evaporated at the start of growth of the barrier layer.
The thickness of the protective layer is an important issue. When the protective layer is too thin, the evaporation of In from the well layer is not prevented. When the protective layer is too thick, carriers are trapped or recombined in the protective layer, thereby reducing emission performance.
Therefore, when forming the protective layer, controllability of the thickness must be improved. Conventionally, the thickness was controlled by making the growth rate of the protective layer extremely lower than that of the well layer. However, this method cannot improve emission performance as expected, rather emission performance was reduced.
The inventors have found, as a result of intensive research to determine the cause, that the crystallinity of an interface between the well layer and the protective layer is deteriorated due to fluctuation of raw material gas flow caused by making the growth rate different between the well layer and the protective layer.